1. Field of the Invention
The present invention relates to a display device having a display screen for displaying text, still images, moving images, and the like. In addition, the invention relates to a technique for improving the visibility of a display screen in various usage environments.
2. Description of the Related Art
In recent years, a so-called self-luminous display device is attracting attention, which has pixels each formed with a light-emitting element such as a light-emitting diode (LED). As a light-emitting element used in such a self-luminous display device, there is an organic light-emitting diode (also called an OLED (Organic Light-Emitting Diode), an organic EL element, an electroluminescence (EL) element, or the like), which has been attracting attention and used for an EL display (e.g., an organic EL display). Since the light-emitting element such as an OLED is a self-luminous type, it is advantageous as compared to a liquid crystal display in that high visibility of pixels is ensured, no backlight is required, high response speed is achieved, and the like. The luminance of a light-emitting element is controlled by the value of current flowing therein.
As a method of controlling gray scales (luminance) in such a display device, there are a digital gray scale method and an analog gray scale method. In the digital gray scale method, gray scales are expressed by controlling on/off of a light-emitting element in a digital manner. On the other hand, as for the analog gray scale method, there is a method of controlling the light-emission intensity of a light-emitting element in an analog manner, or a method of controlling the light-emission time of a light-emitting element in an analog manner.
In the digital gray scale method, only two states of a light-emitting element can be selected, which are a light-emission state and a non-light-emission state; therefore, only two gray scales can be expressed. Thus, the digital gray scale method is often used in combination with another method to achieve multi-gray scale display. As a method for achieving multi-gray scales, a time gray scale method is often used in combination (see Patent Documents 1 and 2).
As examples of a display which expresses gray scales by digitally controlling a light-emission state of pixels in combination with the time gray scale method, there are an organic EL display using a digital gray scale method, a plasma display, and the like.
The time gray scale method is a method of expressing gray scales by controlling the length of light-emission periods or the number of light-emitting operations. That is, one frame is divided into a plurality of subframes, and each subframe is weighted in the number of light-emitting operations, the length of light-emission periods, or the like, so that the total weight (the sum of the light-emitting operations or the sum of the light-emission periods) is varied between different gray scales, in order to express each gray scale.
Thus far, such display panels have been required to provide high image quality, and display panels having functions of automatically or manually adjusting brightness or contrast have been in widespread use. For example, there is a liquid crystal display device having a function of adjusting visibility of gray scales by changing the transmissivity of liquid crystals (see Patent Document 3).    [Patent Document 1] Japanese Patent Laid-Open No. 2001-324958    [Patent Document 2] Japanese Patent Laid-Open No. 2001-343933    [Patent Document 3] Japanese Patent Laid-Open No. 2003-186455
However, although a liquid crystal panel exhibits excellent visibility in the indoor environment with an illuminance of 300 to 700 lx, it has a problem in exhibiting a significantly low visibility in the outdoor environment with an illuminance of 1,000 lx or higher. There is a panel called a reflective liquid crystal panel which has a structure where the outside light is reflected by a pixel electrode; however, it has low image quality under an indoor fluorescent lighting; therefore, it cannot solve the essential problem. That is, it has been impossible to ensure high visibility in various environments in the range from, for example, dark places or indoors (e.g., under a fluorescent lighting) to outdoors (e.g., under the sunlight).